This invention relates in general to a method and apparatus for installing and supporting an electrical submersible pump cable, and in particular to helically wound coil springs affixed to an electrical submersible pump cable for engaging an inside wall of coiled tubing.
Electrical submersible pumps (ESP) are normally installed on jointed production tubing and powered by an ESP cable attached to the outside of production tubing. All produced fluids are pumped up the production tubing to the surface.
Oil well completions are being developed to deploy ESPs on the bottom of continuous coiled tubing where the power cable is placed inside the coiled tubing. In these installations, produced fluids are pumped up the annulus between the coiled tubing and the production tubing, or well casing or liner. Many advantages are gained through the use of coiled tubing such as faster deployment, the elimination of a need for large workover rigs, and less frictional pumping losses.
Because ESP cable cannot support its total vertical weight, cable support must be provided by the coiled tubing at regular intervals. Various proposals have been made to provide support, such as the use of mechanical anchors. A need exists for anchors which can be used in fairly small diameter coiled tubing, which will accommodate movement associated with thermal expansion and which will accommodate bending of coiled tubing.
This invention discloses a new method for installing and supporting an ESP cable in a coiled tubing while allowing slight side-to-side or reciprocal movement of the electrical power cable with respect to the tubing as may occur during handling and service. Helically wound coil springs are wrapped around and attached to a cable at regular intervals. The upper section of each coil spring or support spring is affixed to the cable armor by a connecting section such as a cable clamp. A remainder of each spring is formed in a long open helix facing downward when the tubing is installed in a well. The open helix section experiences frictional engagement between the spring and the coiled tubing during installation. The entire surface of the spring may be knurled to increase friction between the spring and the coiled tubing. Additionally, the spring may be provided with a square cross-section to increase the surface area in contact with the coiled tubing. The open helix terminates in a free end. Preferably, the free end is sharpened and hardened for frictional engagement with the inner surface of the tubing.
In one embodiment, the open helix portion of the spring is divided into two sections. The first and longest section is referred to as the undersized section and has a diameter just slightly smaller than the inside of the coiled tubing. The second section located at the very bottom of the helical spring is designed to be slightly larger than the inside diameter of the coiled tubing and is referred to as the oversized section.
A sufficient number of support springs are placed on the cable prior to installation in the tubing. Preferably, the cable is installed in the tubing by pulling the cable in a direction toward the connecting section of the spring. When each spring enters the tubing, the open helix portion of each spring engages the inside diameter of the tubing wall, causing a dragging force. This force stretches the spring, causing the oversized open helix section of the spring to increase in length and decrease in diameter until the stretched spring slides inside the tubing. The process is repeated with each spring support as the entire cable is pulled into the tubing.
When the coiled tubing and cable assembly is placed in a vertical position during installation, the cable is pulled downward by gravity. The outside diameter of the spring, which is preferably knurled, engages the inner surface of the tubing. Movement of the cable in the downward direction with respect to the tubing causes the support springs to compress or shorten due to frictional forces between the spring and the inside surface of the tubing wall. As the helical springs are compressed, the outside diameter of the springs expand to cause a self regulating locking action between the tubing and the spring while preventing the cable from moving downward. The locking action increases with an increased downward force on the cable so as to make a xe2x80x9cfail safexe2x80x9d attachment between the cable and the tubing. It has been found that vibration of the coiled tubing, such as may occur during installation, may cause the springs to initially slip within the coiled tubing. However, after the initial slippage, equilibrium is achieved.